High-speed switching is required in many general purposes industrial control applications. Typically in such cases reed relays or other suitable relays have been employed to open and close the switch contacts and to provide electrical isolation between the control and power circuits. Relay life is adversely affected, however, by arcing which tends to occur across the contacts as they begin to open. While this problem is reduced somewhat by the use of larger contacts, such contacts operate relatively slowly and are often unsuitable for use in many high-speed applications.
A number of devices have attempted to reduce the arcing problem by measuring a zero-current crossing point of the load current and timing the contacts to begin opening at the next zero-current crossing. In addition to requiring complex and relatively slow timing circuitry these devices often employ large, heavy contacts. As a result they tend to operate much too slowly for use in equipment requiring rapid, for example 1-10 millisecond, switching speeds. Moreover, because these devices predict the subsequent zero-current crossing their operation may be disrupted if the frequency of the power supply changes and the subsequent zero-current crossing is different than predicted.
Certain other devices employ zero-current crossing detection in connection with solid state switches which are connected in the load circuit. Again, these devices tend to be complex and many operate rather slowly. Moreover, they exhibit a certain degree of leakage through the solid state switch and, as a result, do not provide optimum electrical isolation between the power supply and load.